Butyl rubber is used in producing pneumatic tire casings because its impermeability to air and other gases prevents the loss of compressed air by diffusion. The practice of lining the interior of tires with a sealant material so as to reduce the deflation of the tire when punctured is well known. A method for lining a tire with such a sealant is disclosed in U.S. Pat. No. 4,418,093. A particularly successful type of tire lining is a tacky, self-healing elastomer such as urethane. When a lined pneumatic tire is penetrated by a sharp object, the elastomer bonds to the surface of the object and thereby closes off any opening through which the pressurized air in the casing can escape. When the object is withdrawn, the tacky walls of the elastomers close upon themselves and fuse, again preventing the escape of the compressed air in the tire.
The desirability of achieving maximum bonding strength between urethane formulations and cured rubber or synthetic rubber substrates has long been known in the art. The pull strength of a urethane-to-rubber bond should be equal to that of rubber-to-rubber or urethane-to-urethane bonds. A desirable bond strength should be equal to the tear strength of the weakest of the two substrates or adherends, and preferably 80 pounds per linear inch (pli). U.S. Pat. No. 4,240,852 assigned to Synair Corporation describes a method of bonding urethane formulations to rubber using primers or adhesive agents, such as .alpha.-cyanoacrylate ethyl ester. In that method, the rubber or synthetic rubber substrate is cleaned and then coated with a liquid adhesive. Before the adhesive has a chance to dry, the substrate surface is coated with a liquid or paste urethane formulation and then allowed to cure. Following the procedure outlined above, it is possible to obtain bond strengths in excess of 80 pli.
A potential concern with the method for bonding urethane to butyl rubber using cyanoacrylate adhesives is their heat instability. Cyanoacrylate adhesives lose bond strength at elevated temperatures, which could limit their use in tires operating in zones of elevated temperature. The temperature inside truck tires operating at high speed on long hauls with full loads can reach 280 degrees Fahrenheit. The object of the present invention is directed to a method of preparing the rubber substrate for adhesion to urethane such that the adhesive does not lose its bond strength at elevated temperatures.